1. Field of the Invention
The invention relates to the preparation of pure melamine by pyrolysis of urea in a high-pressure process and purification of the resulting melamine melt by divided stripping.
2. Description of the Prior Art
In high-pressure processes for preparing melamine, urea is reacted in an endothermic liquid-phase reaction to give melamine. In this reaction, 3 mol of CO2 and 6 mol of NH3 are formed per mole of melamine, and the liquid melamine contains, depending on pressure and temperature conditions in the reactor, additional varying amounts of dissolved NH3 and CO2 and also high molecular weight and low molecular weight by-products and unreacted urea.
It is known from U.S. Pat. No. 3,116,294 that the CO2 can be removed from the crude melamine by countercurrent stripping of a crude melamine melt with gaseous NH3. In WO 00/21940, it is stated that the stripping is advantageously carried out in a column which is filled with liquid melamine and not in a gas-filled column. Furthermore, it is possible to cool the melt in the bubble column to a temperature which is 5–20° C. above the melting point of melamine.